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Aerosols play an important part in the radiative balance of the atmosphere. This influence is exerted in two ways:

  1. Some aerosols, in particular sulphate particles, have a direct effect through reflection of sunlight back into space
  2. Aerosols have an indirect effect through their role as cloud condensation nuclei

The long term effects of aerosols on climate forcing are still not fully understood and we have only a limited knowledge of their spatial and temporal distribution and variability.

A network of ground based photometers has been set up to measure aerosol properties at selected sites, but this does not provide anything like global coverage as illustrated in the map below.

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AERONET sites around the world

The AERONET (AErosol RObotic NETwork) program is an inclusive federation of ground-based remote sensing aerosol networks established by AERONET and PHOTONS and greatly expanded by AEROCAN and other agency, institute, and university partners. The goal is to assess aerosol optical properties and validate satellite retrievals of aerosol optical properties. The network imposes standardization of instruments, calibration, and processing. Data from this collaboration provides globally distributed observations of spectral aerosol optical depths, inversion products, and precipitable water in geographically diverse aerosol regimes.

 

A comprehensive global aerosol climatology can be developed only by using satellite data.

To do this, we must exploit some feature of top-of-atmosphere radiance fields that is sensitive to aerosol type and concentration. Aerosol interaction with electromagnetic radiation is wavelength dependent, and the degree of scattering depends strongly on the angle between the solar beam and the direction of sampling.

The methodolgy used in the CHRIS-PROBA project is therefore to sample top-of-atmosphere radiation in appropriate parts of the spectrum, and from different directions nearly simultaneously. Aerosol reflectance depends on the extinction coefficient. This in turn depends on the aerosol particle size distribution and on the wavelength of the light. In figure 5 the wavelength dependence of the extinction coefficient is shown for different aerosol effective radii.

The aerosol properties will be retrieved by inversion of radiative transfer models.  The enhanced spatial, directional and spectral sampling of the CHRIS-PROBA system promises further improvements.

Validation of the method will be made against data from sites in the AERONET system of sun photometers. Two sites are selected for each of the major aerosol types (desert, maritime, urban and continental/rural), chosen to give a good geographical spread. The experiment is designed to last for a whole year's observations, in order to sample the full seasonal variability of aerosol properties.